A surface light source device utilizing a light guide plate made of light scattering and guiding material and a prism sheet has been proposed and applied to back lighting for a liquid crystal display and the like.
The prism sheet is a sheet-like member made of optical material, which has a surface provided with a large number of V-shaped groove rows, that is, a prism surface. It is known that this element has a function to alter the propagation-directional characteristics of luminous flux.
FIG. 1 shows the most general usage of a prism sheet in a surface light source device provided with a light guide plate made of light scattering and guiding material. Referring to FIG. 1, a light guide plate 1 made of light scattering and guiding material has a wedge-shaped section. The light scattering and guiding material may be obtained by uniformly distributing a substance of different refractive index in a matrix made of polymethyl methacrylate (PMMA), for instance.
The side end surface on the thick side of the light guide plate 1 serves as an incidence surface 2 and a light source element (a fluorescent lamp) L is disposed in the vicinity of the incidence surface.
A reflector 3 is disposed along one surface (a back surface 6) of the light guide plate 1. For the reflector 3, a regular-reflective silver foil sheet or a diffuse-reflective white sheet is used. An illumination flux is emitted from the other surface (an exiting surface 5) of the light guide plate 1. A prism sheet 4 is disposed on the outside of the exiting surface 5.
In FIG. 1, the gap between the prism sheet and the light guide plate 1, the pitch of prism rows and the depth of prism and others are exaggerated for the sake of illustration. One surface of the prism sheet 4 is composed of V-shaped prism surfaces 4a, 4b, and the other surface is a flat surface 4e.
In the prism sheet 4 disposed as shown in FIG. 1, the flat surface 4e faces the outside and serves as a luminant surface 4e, from which an illumination flux 4f is emitted. If a well-known liquid crystal display panel is disposed further outside the prism sheet 4, a liquid crystal display of back lighting type is constituted.
As the result of repetitive reflection occurring in the wedge-shaped section of the light guide plate 1, such a surface light source device shows excellent characteristics for the efficiency of light utilization and the uniformity in luminance.
Light introduced from the light source element L into the light guide plate 1 is guided toward an end surface 7 on the thin side, while being affected by a scattering action and a reflecting action in the light guide plate 1. In the process, the light is emitted little by little from the exiting surface 5. The light emitted from the exiting surface 5 has directivity according to the size of particles of different refractive index distributed in the light guide plate 1 (more generally speaking, according to the correlation distance in a structure having uneven refractive index). In other words, the illumination flux emitted from the exiting surface 5 assumes a parallel luminous flux.
The larger the size of the particles of different refractive index distributed in the light guide plate 1 is (more generally speaking, the more the correlation distance is), light emitted from the exiting surface 5 is parallelized more clearly. The preferential propagation direction (the main propagation direction of illumination flux) is generally inclined at an angle ranging from about 25.degree. to 300.degree. with respect to the exiting surface as viewed from the side of the incidence surface 2.
On the basis of the above fact, a description will now be given on a function to alter the propagation directional characteristics of the prism sheet 4 in the conventional surface light source device with reference to FIGS. 2 and 3.
FIG. 2 is a view for explaining the behavior of light on a section along "a longitudinal direction" in the structure shown in FIG. 1. In FIG. 2, "a longitudinal direction" means a direction parallel to a light supply direction toward the light guide plate 1, in other words, a direction perpendicular to a running direction of the incidence surface 2. On the other hand, "a transverse direction" means a direction perpendicular to the light supply direction toward the light guide plate 1, that is, the running direction of the incidence surface 2.
As shown in FIG. 2, the prism sheet 4 is disposed along the exiting surface 5 of the light guide plate 1 50 that the prism surface is directed inward. An vertical angle .phi.3 of each prism element on the prism surfaces is preferably about 600.degree..
As described above, when the light supply direction is expressed by an arrow L', the preferential propagation direction of luminous flux emitted from the exiting surface 5 is inclined at an angle .phi.2 of about 60.degree. with respect to a normal extending from the exiting surface 5. When a refractive index of the light guide plate 1 (PMMA matrix) is assumed to be 1.492, the angle .phi.1 of incidence with respect to the exiting surface 5 to provide the angle .phi.2 of about 60.degree. is about 35.degree.. A beam which meets such conditions is called a typical beam. In FIG. 2, the typical beam is denoted by a reference numeral B1.
The typical beam B1 emitted from the exiting surface 5 makes a straight propagation through an air layer AR (having a refractive index n0 of approximately 1.0), and thereafter, is incident on the prism surface 4a of the prism sheet 4 at an angle (.phi.3 of about 60.degree.) close to a perpendicular. It is to be noted that the quantity of light incident on the prism surface 4b on the opposite side is relatively very small.
Subsequently, the typical beam B1 makes a substantially straight propagation through the prism sheet 4 up to the prism surface 4b to be regularly reflected toward the flat surface 4e of the prism sheet 4, impinging on the flat surface 4e at an angle close to a perpendicular to be emitted to the outside. The preferential propagation direction of luminous flux emitted from the exiting surface 5 is altered into a direction substantially perpendicular to the exiting surface 5 through the above process.
However, it is to be noted that the altered preferential propagation direction is not always accurately perpendicular to the exiting surface 5. Namely, the altered preferential propagation direction may be adjusted within a range of angles to some extent through design of the vertical angle .phi.3, material (refractive index) of the prism sheet 4 and material (refractive index) of the light guide plate 1 and others.
FIG. 3 is a sectional view for explaining the behavior of light in another conventional structure utilizing the prism sheet 4. Compared with the structure shown in FIGS. 1 and 2, the prism sheet 4 is reversed so that the prism surface face is directed inwardly.
The vertical angle .phi.4 of each prism element consisting of the prism surfaces may be about 70.degree., for instance. In the structure, in which the prism surfaces face outside, an vertical angle to provide desired effects ranges more widely than that in the structure, in which the prism surfaces face inside.
When the light supply direction is expressed by an arrow L', a typical beam B2 is incident on the exiting surface 5 at an angle .phi.1 of about 35.degree., and most of the typical beam is emitted to an air layer AR (having a refractive index n0 of approximately 1.0), similarly to the case of FIG. 2. An angle .phi.2 of emission comes to about 60.degree..
The typical beam B2 makes a straight propagation through the air layer AR and is diagonally incident on the flat surface 4e of the prism sheet 4, making propagation through a refractive path as shown in FIG. 3. Then, the typical beam is emitted from the surface 4c of the prism sheet 4 at an angle close to a perpendicular to the exiting surface 5. It is to be noted that the ratio of beam emitted from the surface 4d is relatively very small. Since the path of light after having been incident on the flat surface 4e varies depending on a refractive index n2 of the prism sheet 4 or a prism vertical angle .phi.4, the preferential propagation direction may be adjusted by adjusting these parameters.
The surface light source devices of such conventional types are excellent in that a thin structure is realized, and that a uniform and bright illumination flux is provided so as to preferentially make propagation in a desired direction.
However, demands on a liquid crystal display of back lighting type have been recently more exacting. Namely, not only a thin structure, a large image area and power-saving properties, but also a visual feeling of high quality has been highly required.
The surface light source devices of conventional types described above do not sufficiently meet such requirements. In particular, the surface light source devices of conventional types do not reach the stage of simultaneously satisfying the level and uniformity of brightness and the sense of softness when the luminant surface (the top surface of the prism sheet) is viewed with the naked eye.
Namely, in the prior art, it is not possible to provide a surface light source device having a luminant surface which is fine and has sufficient whiteness without glaring. Further, irregularities in luminance called a reflective appearance occurs in the exiting surface in the vicinity of the incidence surface, resulting in degradation of display quality.
It is supposed that visual quality is degraded due to the causes as follows. Generally, scattering power of the light guide plate 1 in the surface light source device shown in FIG. 1 is not intensive enough to allow clear observation. The reason is that such enough intensive scattering power given to the light scattering and guiding material, the light scattering and guiding material reduces light guiding capacity and is hard to meet the requirements that the luminance of the luminant surface should be uniform. This tendency becomes more remarkable according as the size of the light guide plate (namely, the size of the luminant surface or the size of an image plane of a liquid crystal display) is increased.
Therefore, as far as it judges from a visual point of view, it is not possible to expect an intensive light diffusing action inside the light guide plate 1. As a result, it is supposed that a remarkable quantity of light reflected from the reflector 3 disposed along the back surface of the light guide plate 1 reaches observer's eyes without undergoing sufficient diffusion.
In other words, the ground of the reflector 3 has a tendency to be seen by the observer's eyes. Accordingly, if a regular-reflective reflector 3 is used, undesirable visual feeling peculiar to a regular reflection surface, that is, insufficiency of so-called "whiteness" or that of so-called "softness", or "glaringness" is caused inevitably.
Further, since less diffused light is transmitted through the prism sheet, rows of prism grooves defined by the prism surfaces may be observed as stripes.
Further, insufficient diffusion of light occurs particularly in a portion close to the incidence surface, and a phenomenon of so-called reflective appearance is caused, resulting in exerting a bad influence on the display quality of a liquid crystal display.
It is supposed that such phenomena and a resultant undesirable sense affecting an observer are caused by not only the level of the light quantity but also a combination of related factors such as color temperature and propagation directional characteristics of illumination flux.
When a diffuse-reflective white sheet is used for the reflector 3, a visual problem is association with "whiteness" is dissolved to some extent. However, when the reflector 3 is formed to have a diffuse-reflective surface, such a reflector exerts a bad influence on the level and uniformity of the general luminance of the luminant surface.
Further, when some irregularities (i.e., local wrinkles or irregularities) are existent on the surface of the reflector 3 in the case where either of the regular-reflective or diffuse-reflective reflector is used, these irregularities are seen through the reflector, and therefore, the degradation of visual quality may occur.
For solving the above problems, the present inventors have previously proposed a structure, in which an additional prism sheet is disposed along the back surface of a light guide plate made of light scattering and guiding material, in addition to a prism sheet disposed along an exiting surface of the light guide plate (PCT/JP96/00561).
According to the above proposal, the additional prism sheet is disposed along the back surface of the light guide plate such that the prism rows run in parallel with the light supply direction. This surface light source device effectively solves the above problems. However, it is unsatisfactory from the viewpoint of a thin structure and the reduction of manufacturing cost that an additional prism sheet is required in addition to the prism sheet disposed along the exiting surface.